Tuesday 31 January 2012 15.08 EST
First published on Tuesday 31 January 2012 15.08 EST

The head of US intelligence has warned that there is an increasing likelihood that Iran could carry out attacks in America or against US and allied targets around the world.

The warning from the director of national intelligence, James Clapper, reflects rapidly rising tensions over Iran's nuclear programme after the US and EU announced embargoes on the Iranian oil trade in the past few weeks, Israel leaked details of its preparation for a possible conflict and both the west and Iran boosted their military readiness in the Gulf.

The US plans to send a third aircraft carrier to the region in March, while Iran's military has threatened to block the entrance to the Gulf in the strait of Hormuz and is planning to hold naval exercises there in the next few weeks involving a host of new weapons.

Presenting his annual "worldwide threat assessment" to Congress, Clapper said an alleged plot to blow up the Saudi ambassador in Washington last year, which the US blamed on the Iran's Revolutionary Guard, "shows that some Iranian officials – probably including the supreme leader Ali Khamenei – have changed their calculus and are now more willing to conduct an attack in the United States in response to real or perceived US actions that threaten the regime."

Clapper added: "Iran's willingness to sponsor future attacks in the US or against our interests abroad probably will be shaped by Tehran's evaluation of the costs it bears for the plot against the ambassador as well as Iranian leaders' perceptions of US threats against the regime."

Western officials say that in the past year there has been a notable increase in activity around the world by suspected members of Iran's Quds force, the external operations arm of its Islamic Revolutionary Guard Corps (IRGC), which they say could reflect positioning of units capable of carrying out reprisal attacks against western and Israeli targets if Iran was itself attacked. "There have been a lot of reports recently of IRGC activity abroad," one western official said. "There is a great deal of worry about the IRGC carrying out covert and deniable actions. But they may be overestimating how much they can hide their role. The US and others are very concerned about this.

"In this situation, there is a risk of miscalculation," the official added, "or of rogue elements operating independently."

US officials say that the alleged Washington bomb plot showed a new recklessness by an increasingly embattled Iranian regime. An Iranian-American was charged last October with planning to blow up the Saudi ambassador to the US while he ate at his favourite Washington restaurant, potentially killing many Americans at the same time.

The US has claimed authorisation for the attack came from the highest levels of the regime, but Clapper's remarks marked the first time Washington has openly blamed the supreme leader.

However, a western official cautioned that there was no evidence a final decision had been taken to go ahead with the attack. "Our understanding is that this was at the stage of operational planning. The order was for everything to be put in place. There was not, as far as I know, a green light," the official said.

In recent days, both the Thai and Azeri governments made a number of arrests of suspects allegedly linked to Iranian intelligence who are accused of planning to kill Israel diplomats and a rabbi. One possibility, western governments believe, is that the plots were intended as reprisals for a string of murders in Tehran of Iranian scientists linked to the country's nuclear programme. Iran has blamed Mossad for the killings, an accusation that many western officials think is plausible.

After an Iranian threat last month to close the strait of Hormuz in response to oil sanctions, the US has deployed two aircraft carriers, the USS Abraham Lincoln and the USS Carl Vinson, in the region. A thirdis scheduled to head to the Gulf in March.

John Pike, a military analyst and the head of the GlobalSecurity.org thinktank said: "That almost never happens. They seldom even have two."

He added that a fourth carrier, the USS John Stennis, was sailing away from the area but at a slow pace and could be back within a few days.

Tensions have been stoked further by leaked details of Israeli military preparations and cabinet deliberations on whether to strike Iran in the next few months, in an effort to set back its nuclear programme by a few years. Western officials confess they are unsure to what extent such reports represent an Israeli bluff to force urgent action by the US and its European allies, but say they do take the Israeli threats seriously.

One possibility is that Israel could launch air strikes at the height of the US presidential election campaign, on the grounds that the Obama administration would have to mute any politically risky criticism of a longstanding US ally.

Some observers believe the planned European and US oil embargoes, due to come into effect five months from now with potentially severe implications for the Iranian economy, along with a military build-up in the region, could themselves raise the risk of miscalculation on all sides.

"I don't think they are playing Iran anything like as well as they think they are," said Richard Dalton, a former British ambassador to Tehran. "The oil embargo tends to give those elements in Iran who want to have maximal defences, including nuclear defences, added weight to their arguments. Also they are poking Iran with a sharp stick but this is not accompanied by a new negotiating incentives."

In a strikingly critical report, an influential Israeli thinktank, the Institute for National Security Studies, warned that the Israeli leadership could be rushing into a decision to attack without properly thinking of the implications. The authors said that Israeli society should "not assume that decision makers will automatically make correct choices based on a rational of an attack's cost effectiveness".

"Past experience has proven that such an in-depth discussion does not always take place," the report said. It questioned whether a nuclear Iran was really an existential threat to Israel and warned that unilateral action would alienate the US and other Israeli allies.

"The image – not the first of its kind – will be of an Israel unilaterally violating the rules of the international game and launching a military campaign without legitimacy from the security council. This might increase Israel's isolation as well contribute to its delegitimisation."

Iran insists its nuclear programme is for peaceful purposes. The west and Israel allege it is intended to give Iran at least the capacity to make a bomb, but Clapper conceded in his remarks : "We do not know, however, if Iran will eventually decide to build nuclear weapons."